Tunable laser wavelength modulation absorption spectroscopy is finding widespread use in various applications. One such application is the quantification of the amount of chemical species (the measurand) in a substance and in particular in an artificial or natural process such as an industrial, medical or physiological process gas analysis where an improved performance may be obtained compared to other techniques.
A typical system consists of a tunable laser source such as a tunable diode laser (TDL) that emits a beam of light that is focussed on a detector. The substance that is to be analysed is positioned between the tunable laser source and the detector, so that the light incident on the detector has been modified by its passage through the substance. The modifications to the light enable various parameters of the measurand to be determined by a signal processing system that is coupled to the detector. In some cases the substance to be analysed is a gas produced by an industrial process, and the measurand may be one or more chemical species that are present in this process gas. Examples of measurand species include but are not limited to gaseous water, O2, CO and CO2 and hydrocarbons such as methane. The presence and/or amount fraction (concentration) of one or more of these measurand species may be determined by absorption spectroscopy measurements using one or more TDLs.
In operation of the laser gas analyser system, the wavelength of the beam emitted by the TDL is scanned over a range of wavelengths including one or more absorption lines of the measurand. At certain specific wavelengths within the range of wavelengths scanned, light is absorbed by the measurand and these spectral absorption lines can be detected by measuring the light transmitted through the substance to be analysed. This allows the necessary spectroscopic information to be acquired to determine not only the amount fraction of the measurand, but also optionally to determine the influence of pressure, temperature or background mixture composition. In some cases it is possible to use a single laser source to measure a plurality of measurands. In these cases, the output wavelength of the laser source is swept across a wavelength range that includes at least one discernable absorption line for each of the plurality of measurands.
In a well-designed system, wavelength modulation techniques offer very high sensitivity and enhanced spectral resolution. In particular, second harmonic wavelength modulation spectroscopy is well suited to gas analysis due to its ability to cope with a wide variety of spectroscopic situations found in industrial processes, such as congested absorption spectra, sensitive trace level measurements and obscured optical transmission.
This is shown by the folloing relationships, where equation [1] represents the Beer-Lambert law of optical absorption, wherein u is the molecular density per unit length of the measurand, I is the detected amount of light, I0 is the incident amount of light (equal to unabsorbed amount when the molecular density is zero), v is the frequency of light and “a” is the absorption coefficient.
                              log          ⁡                      [                                          I                ⁡                                  (                  v                  )                                                                              I                  0                                ⁡                                  (                  v                  )                                                      ]                          =                  -                      u            .                          a              ⁡                              (                v                )                                                                        [        1        ]            
The change in the amount of light detected at any particular frequency (∂I(v)) is related to the molecular density change (∂u) by differentiating equation [1] and is given by equation [2].
                              ∂          u                =                                            ∂                                                I                  0                                ⁡                                  (                  v                  )                                                                                    a                ⁡                                  (                  v                  )                                            ⁢                                                I                  0                                ⁡                                  (                  v                  )                                                              -                                    ∂                              I                ⁡                                  (                  v                  )                                                                                    a                ⁡                                  (                  v                  )                                            ⁢                              I                ⁡                                  (                  v                  )                                                                                        [        2        ]            
Equation [2] shows that if other ambient conditions are stable or corrected for, the change in detected intensity will be proportional to the change in molecular density (∂u), but the detected intensity is also affected by any variations in the amount of incident light (∂I0(v))
Variations in incident amount of light may be caused by a number of factors other than absorbing molecular density changes. For example, variations can be caused by intrinsic fluctuations in the laser output, changes in ambient light intensity levels and/or obscuration in the process sample stream, which may be caused by any combination of dust, tar, corrosion or optical beam misalignment. Obscuration and changing of the intensity of ambient light are to be expected in a furnace. If the variation in incident light is not corrected, this will result in a measurement uncertainty (error) in the processed measurand concentration. Techniques have been developed to deal with these sources of uncertainty, such as described in published patent application GB2524725 (Kovacich et al), which is incorporated herein by reference.
However, there is another potential cause of fluctuations in the optical detector signal, which is not due to direct fluctuations in the ambient light or laser output signal, but due to constructive and destructive interference occurring and causing an oscillation in the detector signal as the laser is scanned across the measurement wavelength range. The use of coherent laser light means that any reflections at any optical surfaces or interfaces along the optical path from the laser output to the detector surface (for example from surfaces/interfaces such as, windows, lenses and reflective interfaces), lead to the production of reflected light with a phase difference in comparison with the incident light, hence leading to optical interference where the light rays interact. The phase relationship between this reflected light and the incident light may change with time due to such factors as temperature, vibration and pressure fluctuations, since these factors may cause physical dimensional, density or refractive index changes.
The detector is integrating this optical interference to produce an intensity signal. Since the phase difference will vary with wavelength along the measure path, the symptoms of this optical interference (or etalons) are typically the production of oscillations on the signal baseline as the laser output is scanned across the wavelength measurement range. These combine with other distortions and cause measurement inaccuracies. The signal “baseline” is the signal that would be seen even if no absorbing signal were present, in other words, the “zero absorption” signal. This baseline signal is superimposed on the actual absorption signal when present. In an ideal world, the baseline would be a straight line (flat line centred at zero in perfect circumstances), but in practice this is never achieved. The baseline may not be perfectly flat across the scan range and may have fluctuations and other distortions (or “noise”), which may be of a random or systematic nature and include the above-mentioned oscillations. These oscillations are also known as “fringe” signals in the case of optical interference. These various distortion effects, of whatever origin, lead to increased uncertainty in the determination of the absorption signal or signals, and hence increased uncertainty in the derived molecular density or concentration of the measurand.
One method to decrease such optical interference is to reduce or eliminate any reflective or partially reflective surfaces in the light path from source to detector that may form etalons, such as by minimising the number of optical components, using wedge windows rather than parallel face windows or anti-reflection coating optimised for the desired wavelength range. However, it is impossible in practice to completely eliminate this interference effect by reducing reflective surfaces. In cases where a multipass cell is used it is unavoidable as the beam path within a multipass cell will always create some amount of optical interference, which is usually significant for trace level measurements.
Another method to reduce the impact of optical interference on the baseline is to measure and record a reference baseline when no measurand is present. This reference baseline may then be subtracted from the live signal to produce a cleaner signal to process. Whilst this may give an immediate improvement to the measurand determination uncertainty, it does not address oscillations on the baseline under changes in ambient conditions (particularly temperature) and hence the effectiveness of this technique is limited.
Another method involves the use of a piezo electric element or similar to oscillate an active optical element such as a lens or mirror in the optical path. This has the effect of continuously varying the optical pathlengths and hence the phase variations and resultant optical interference. This results in blurring or smoothing down the sinusoidal oscillation on the baseline, through integration over time of the interference fringes formed and therefore reducing the overall effect. However, it adds complexity, cost, suffers from a number of problems due to using a moving element, such as reduced component lifetime and mechanical failure and, in practice, does not eliminate the problem completely. Moreover, most piezo electric elements require a sufficiently high voltage supply that makes operation in flammable hazardous areas unsuitable.
In addition, there are other potential causes of fluctuations in the baseline signal. These may be optical, such as due to ambient light generation or scattered light, or non-optical, such as caused by electromagnetic interference or random or systematic noise. Electromagnetic interference may be short term or persistent.
Hence, there remains a need for an absorption spectroscopy gas analyser system that can produce highly accurate measurements, despite fluctuations in the baseline signal due to optical interference or other effects, which are changing with time. There is also a need for such an analyser system that is able to produce highly accurate measurements in a harsh environment as may typically be found in many industrial processes, such as in a furnace or furnace exhaust pipe.
Note that although the detailed explanations and systems that follow illustrate use of the invention for second harmonic (2f) wavelength modulation spectroscopy for detection and measurement, the novel technique described in this patent specification is applicable to any harmonic absorption measurement, i.e anything from 1st harmonic (direct absorption) to second or higher order harmonics.